This invention relates generally to optical transmission systems and, more specifically, to low insertion loss optical couplers.
Optical couplers are optical transmission system components used to connect planar arrangements of waveguides. As shown in FIG. 1, a star coupler 100 (discussed in detail in U.S. Pat. No. 4,904,042, issued Feb. 27, 1990 to Dragone, and herein incorporated by reference) has a free space region 110 bounded by an input junction 102 for receiving incoming signal(s) from a plurality of individual input waveguides 106 and an output junction 104 for power splitting the input signal(s) and/or coupling portions of the signal(s) to a plurality of individual output waveguides 108. Insertion loss (a reduction in the power of a signal propagating through the coupler 100) typically occurs at the input junction 102 because of the abrupt change in the physical dimensions of the individual input waveguides as compared to the free space region 110. The abrupt change causes a scattering of light associated with the signal, subsequently reducing power.
In a theoretically ideal coupler, waveguides approaching a free space region are nearly parallel to one another. The waveguides are shaped so that they are narrow at first and then increase in width until the gap between them is zero (a point which defines for example, the input junction). Zero gap width along with non-converging waveguides provides for a theoretical insertion loss of zero.
As shown in FIG. 2, the input junction 102 to the free space region 110 of conventional star coupler 100 is essentially arc-shaped. A first individual waveguide 1061 and a second individual waveguide 1062 are shown in detail as having respective uniform waveguide sections 202 and horn sections 204. The uniform waveguide sections 202 have a substantially uniform waveguide width Wu. The horn sections 204 change to a different width Wt as they extend from the uniform waveguide sections 202 to the input junction 102. The radial nature of the geometry of the individual waveguides along the input junction 102 forces them to converge too quickly for the transition to be gradual and thus truly have zero loss. Additionally, the limitations of lithography (a process used to fabricate coupler 100) create a non-zero gap 206 between the first and second individual waveguides 1061, 1062. Even if lithography permitted a zero gap, an average value of this gap 206 as the waveguides converge would be finite which is counter to the desired theoretical zero value gap (i.e., the waveguides are overtly non-parallel); thus, creating the insertion loss condition.
These and other deficiencies of the prior art are addressed by the present invention of an apparatus for optical coupling having a first array of individual waveguides optically communicating with a free space region at a first junction, each waveguide having a tapered region proximate the junction where a gap spacing between tapered regions of adjacent individual waveguides is substantially constant. In one embodiment, the tapered region of each individual waveguide has a length D of approximately 250 xcexcm. The gap spacing is a minimum amount of space between adjacent individual waveguides and in one embodiment of the invention in the range of approximately 1.5-3.5 xcexcm. The first array of individual waveguides also has a horn region immediately proximate the tapered region and opposite the first junction with a changing waveguide width. The first array of individual waveguides also has a waveguide region immediately proximate the horn region and opposite the tapered region with a width that is nearly uniform along its entire length. In one embodiment, the width of the tapered region is decreased (tapers) as a function of length as it extends away from the horn region. The apparatus may also have a second array of individual waveguides communicating with the free space region at a second junction.